Wide-bandgap High Electron Mobility Transistors (HEMTs) offer thermal and chemical stability, as well as high mobility and breakdown field, but the standard HEMT is a depletion mode device and enhancement mode (normally-off) devices are desirable for enhancement mode transistor of enhancement mode/depletion mode logic circuits mixed-signal (analog/digital), and high power switching applications to reduce power consumption and simplify circuit design and safety. In addition, recessed gate device structures with well controlled barrier material layer thickness are desired for depletion mode microwave devices to reduce the source and drain access resistance. Shifting the threshold voltage to create an enhancement mode device has been attempted by etching AlGaN barrier material layer to be sufficiently thin to reduce the polarization induced two dimensional electron gas channel charge or by implanting negative charge in the AlGaN barrier material layer under the gate by exposing the device to a fluorine-based plasma, but plasma exposure in these approaches risks damage and thus requires complicated post-processing anneal operations. In addition, the process of etching the AlGaN barrier material layer to be sufficiently thin to achieve enhancement mode (normally-off) operation results in a low polarization induced channel charge in the source and drain regions and thus high source and drain access resistance. Accordingly, there is a need for improved normally-off (enhancement mode) high electron mobility transistors and recessed gate depletion mode microwave transistors and techniques for fabricating such transistors.